1. Field of the Invention
The present invention relates to an apparatus and method for probing integrated circuits using laser illumination, wherein reflected laser light is modulated by switching of an active device under test (DUT).
2. Description of the Related Art
Probing systems have been used in the art for testing and debugging integrated circuit (IC) designs and layouts. Various laser-based systems for probing IC's are known in the prior art. While some description of the prior art is provided herein, the reader is encouraged to also review U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545, which are incorporated herein by reference in their entirety. Additional related information can be found in Yee, W. M., et al. Laser Voltage Probe (LVP): A Novel Optical Probing Technology for Flip-Chip Packaged Microprocessors, in International Symposium for Testing and Failure Analysis (ISTFA), 2000, p 3-8; Bruce, M. et al. Waveform Acquisition from the Backside of Silicon Using Electro-Optic Probing, in International Symposium for Testing and Failure Analysis (ISTFA), 1999, p 19-25; Kolachina, S. et al. Optical Waveform Probing—Strategies for Non-Flipchip Devices and Other Applications, in International Symposium for Testing and Failure Analysis (ISTFA), 2001, p 51-57; Soref, R. A. and B. R. Bennett, Electrooptical Effects in Silicon. IEEE Journal of Quantum Electronics, 1987. QE-23(1): p. 123-9; Kasapi, S., et al., Laser Beam Backside Probing of CMOS Integrated Circuits. Microelectronics Reliability, 1999. 39: p. 957; Wilsher, K., et al. Integrated Circuit Waveform Probing Using Optical Phase Shift Detection, in International Symposium for Testing and Failure Analysis (ISTFA), 2000, p 479-85; Heinrich, H. K., Picosecond Noninvasive Optical Detection of Internal Electrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits. IBM Journal of Research and Development, 1990. 34(2/3): p. 162-72; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(16): p. 1066-1068; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Erratum to Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(26): p. 1811; Heinrich, H. K., et al., Measurement of real-time digital signals in a silicon bipolar junction transistor using a noninvasive optical probe. IEEE Electron Device Letters, 1986. 22(12): p. 650-652; Hemenway, B. R., et al., Optical detection of charge modulation in silicon integrated circuits using a multimode laser-diode probe. IEEE Electron Device Letters, 1987. 8(8): p. 344-346; A. Black, C. Courville, G Schultheis, H. Heinrich, Optical Sampling of GHz Charge Density Modulation in SIlicon Bipolar Junction Transistors Electronics Letters, 1987, Vol. 23, No. 15, p. 783-784, which are incorporated herein by reference in their entirety and Kindereit U, Boit C, Kerst U, Kasapi S, Ispasoiu R, Ng R, Lo W, Comparison of Laser Voltage Probing and Mapping Results in Oversized and Minimum Size Devices of 120 nm and 65 nm Technology, Microelectronics Reliability 48 (2008) 1322-1326, 19th European Symposium on Reliability of Electron Devices, Failure Physics and Analysis (ESREF 2008).
As is known, during debug and testing of an IC, a commercially available testing platform, such as, e.g., Automated Testing Equipment, also known as an Automated Testing and Evaluation (ATE) tester, is used to generate test patterns (also referred to as test vectors) to be applied to the IC device under test (DUT). Various systems and methods can then be used to test the response of the DUT to the test vectors. One such method is generally referred to as laser voltage probing (LVP). When a laser-based system such as an LVP is used for probing, the DUT is illuminated by the laser and the light reflected from the DUT is collected by the probing system. As the laser beam strikes the DUT, the laser beam is modulated by the response of various elements (switching transistors) of the DUT to the test vectors. This has been ascribed to the electrical modulation of the free carrier density, and the resultant perturbation of the index of refraction and absorption coefficient of the material of the IC, most commonly silicon. Accordingly, analysis of the reflected light provides information about the operation of various devices in the DUT.
FIG. 1 is a general schematic depicting major components of a laser-based voltage probe system architecture, 100, according to the prior art. In FIG. 1, dashed arrows represent optical path, while solid arrows represent electronic signal path. The optical paths represented by curved lines are generally made using fiber optic cables. Probe system 100 comprises a laser source which, in this particular example, is a dual laser source, DLS 110, an optical bench 112, and data acquisition and analysis apparatus 114. The optical bench 112 includes provisions for mounting the DUT 160.
A conventional ATE tester 140 provides stimulus signals 142 to and receives response signals from the DUT 160 and may provide trigger and clock signals, 144, to the time-base board 155. The signal from the tester is generally transferred to the DUT via test boards, DUT board (adapter plate) and various cables and interfaces that connect all of these components. Generally, the ATE and the LVP systems are produced and sold by different and unrelated companies. Thus, the reference to the description of embodiments of the inventive system relate only to the LVP, and not to the ATE. That is, the ATE is not part of the probe system 100.
Turning back to the probe system 100, the time-base board 155 synchronizes the signal acquisition with the DUT stimulus and the laser pulses. Controller 170 controls as well as receives, processes, and displays data from the signal acquisition board 150, time-base board 155, and the optical bench 112.
The various elements of probe system 100 will now be described in more detail. Since temporal resolution is of high importance in testing DUT's, the embodiment of FIG. 1 utilizes prior art pulsed lasers, wherein the laser pulse width determines the temporal resolution of the system. Dual laser source 110 consists of two lasers: a pulsed mode-locked laser, MLL 104, source that is used to generate 10-35 ps wide pulses, and a continuous-wave laser source, CWL 106, that can be externally gated to generate approximately 1 us wide pulses. The MLL 104 source runs at a fixed frequency, typically 100 MHz, and must be synchronized with the stimulus signals 142 provided to the DUT 160, via a phase-locked loop (PLL) on the time-base board 155, and the trigger and clock signals 144 provided by the ATE tester. The output of the DLS 110 is transmitted to the optical bench 112 using fiber optics cable 115. The light beam is then manipulated by beam optics 125, which directs the light beam to illuminate selected parts of the DUT 160. The beam optics 125 consists of a Laser Scanning Microscope (LSM 130) and beam manipulation optics (BMO 135). The specific elements that are conventional to such an optics setup, such as objective lens, etc., are not shown. Generally, BMO 135 consists of optical elements necessary to manipulate the beam to the required shape, focus, polarization, etc., while the LSM 130 consists of elements necessary for scanning the beam over a specified area of the DUT. In addition to scanning the beam, the LSM 130 has vector-pointing mode to direct and “park” the laser beams to anywhere within the field-of-view of the LSM and Objective Lens. The X-Y-Z stage 120 moves the beam optics 125 relative to the stationary DUT 160. Using the stage 120 and the vector-pointing mode of the LSM 130, any point of interest on the DUT 160 may be illuminated and probed.
For probing the DUT 160, the ATE 140 sends stimulus signals 142 to the DUT, in synchronization with the trigger and clock signals provided to the phase-locked loop on the time-base board 155. The phase-lock loop controls the MLL 104 to synchronize its output pulses to the stimulus signals 142 to the DUT. MLL 104 emits laser pulses that illuminate a particular device of interest on the DUT that is being stimulated. The reflected light from the DUT is collected by the beam optics 125, and is transmitted to photodetector 138 via fiber optic cable 134. The reflected beam changes character (e.g., intensity) depending on the reaction of the device to the stimulus signal.
Incidentally, to monitor incident laser power, for purposes of compensating for laser power fluctuations, for example, optical bench 112 provides means to divert a portion of MLL 104 incident pulse to photodetector 136 via fiber optic cable 132.
The output signal of the photosensors 136, 138, is sent to signal acquisition board 150, which, in turn, sends the signal to the controller 170. By manipulation of the phase lock loop on the time-base board 155, controller 170 controls the precise time position of MLL 104 pulses with respect to DUT 160 stimulus signals 142. By changing this time position and monitoring the photosensors signals, the controller 170 can analyze the temporal response of the DUT to the stimulus signals 142. The temporal resolution of the analysis is dependent upon the width of the MLL 104 pulse.
It is also known in the art to perform continuous wave LVP, wherein a continuous wave laser is used to illuminate a device on the DUT and the continuously reflected light is collected. The continuously reflected light contains timing information relating to the response, i.e., switching, of the active device to various stimulus signals. The reflected light signal is continuously converted into electrical signal by a photodetector, e.g., avalanche photodiode (APD), and is amplified. The timing information is contained within the electrical signal and represents detected modulation of the device, which can then be displayed in either the time-domain using an oscilloscope or in the frequency domain using a spectrum analyzer.
When using a CW laser for LVP, the beam is focused onto an active transistor inside a silicon wafer. It happens that the amount of light reflected from the transistor contains is a function of the voltage across the transistor, hence the reflected laser beam carries superimposed on it an amplitude-modulated signal, the envelope of which duplicates the electrical waveform being probed. The reflected light is captured by a photodetector, which converts the amplitude-modulated optical signal back into a copy of the on-silicon voltage waveform. For reference, the change in reflectivity is relatively small, and a typical value may be only 100 parts per million (ppm). Nevertheless, with a low-noise, high-speed detector, and a very high speed oscilloscope, it is possible to reconstruct multi-gigahertz waveforms using this optical probing technique. This technique may be referred to as the “slow laser, fast detector” option, because the laser in this case is CW (i.e. very slow) and the detector and electronics must be very fast.
Conversely, when using the MLL technique, a very fast laser is employed, but with relatively slow electronics. The laser used is a mode-locked laser, which outputs a series of, e.g., 5-ps FWHM pulses at a 100 MHz repetition rate. The optoelectronic detection mechanism may be the same as for the CW, but since the pulse is localized in time to only five picoseconds, each MLL optical pulse takes a very high-speed (˜5 ps) sampling of the waveform. Since the pulses only occur every 10 nanoseconds (for a 100 MHz laser) the detection electronics can be relatively slower than the 5-ps optical pulses. In fact, a pulse-picking scheme can be implemented that only lets a single 5-ps optical pulse through, so the detector need only register the brightness of that one pulse every tester loop period, effectively making the detection scheme very slow.
The hidden complexity in both of these schemes is that the signal-to-noise ratio (SNR) of the measurements is very low—typically much less than one—owing to the combination of a weak reflection from the DUT and the relatively low modulation level (about 100 ppm). As a result, many thousands or millions of repeat measurements must be made to bring the SNR level up to a useful level (SNR 10, for instance). Other problems exist for each of these schemes, as follows.
The CW detection scheme starts when the amplitude-modulated CW laser beam reflected from the DUT hits an infrared detector. Typically the intensity of the laser at the detector is 50 to 100 μW, amplitude modulated with a 100 to 500 ppm signal. The biggest two noise sources are the detector's noise-equivalent power (NEP) and shot noise of the laser beam itself, with the former being slightly larger than the latter. The NEP of the detector is about 10 pW/√Hz, and with a good laser the noise of the laser itself is only slightly above shot noise. This gives an output signal, which typically has an SNR<1. It also gives an output signal that has too low of an amplitude to be sent straight into a digitizing oscilloscope, so the detector needs to be followed with an RF amplifier. Fortunately the noise coming out of the detector is so great that the noise contribution of the RF amplifier stage is insignificant. It is worth noting that although the noise of this amplifier is insignificant, the impact of the amplifier's bandwidth on measured signal rise times is significant, and for this reason it would be ideal to eliminate this amplifier from the system.
Typically an oscilloscope is used to sample the reflected waveform. Since the SNR of the waveform is less than one, many waveforms must be averaged together to arrive at a decent output signal. This step is handled by the oscilloscope, which synchronously captures data from repeated tester loops, averages them, and reports a single output that is the modulated waveform.
The main challenge with MLL scheme has to do with allowing only one MLL optical pulse onto the device under test (DUT) during each tester loop cycle. The optical pulse is amplitude-modulated by the DUT, in the same manner as for CW, and returns to a detector. The limitations of the system are the detector noise and the shot noise of the laser. One subtlety of the MLL scheme has to do with the nature of pulsed light. If the average intensity of a CW and an MLL laser are the same, but the power of the MLL pulses is confined in duration to only 5 ps FWHM, then the intensity during those 5 ps is much larger than the CW intensity, which is spread uniformly over time. For instance, if the MLL laser puts out all of it's power in only 5 ps, with the remaining 9995 ps of each 10-ns cycle being dark, then the peak intensity of the MLL beam is approximately 2000 times stronger than the CW beam. It turns out that each MLL measurement thus captures about 2000 times as much information as the CW scheme does, but the compensating tradeoff is that MLL only sampled one 5-ps snapshot of a waveform, while the CW system captured the entire waveform. Thus, it will take 2000 MLL measurements to fill in the 10-ns waveform, whereas the CW can use this time to average 2000 10-ns waveforms in their entirety. These two effects turn out to be a wash, lending no advantage to one method over the other.
Recently the technology of laser voltage imaging has been developed to provide a two-dimensional gray-scale image correlating to voltages at different points in an area of the DUT. More specifically, an LSM is used to raster-scan an area of the DUT and at each point within the area the reflected light, signal is collected and provides a single data value. That is, rather than providing the spectra over a range of frequency band, at each point the amplitude of the signal at a particular frequency spectrum is obtained from the spectrum analyzer. In practice, the spectrum analyzer is set to extract a single frequency of interest (called zero-span), and to provide an output value that is directly proportional to the strength of the received signal at that frequency. Consequently, as the LSM scans the selected area of the DUT, if there is no activity at the frequency of interest, the spectrum analyzer provides low or no output, while if there is activity at that frequency, the spectrum analyzer provides high output. That is, the spectrum analyzer provides an output signal whose amplitude is proportional to the strength of the signal at the selected frequency of interest. This output can be used to generate a map of the scanned area, showing gray-scale levels corresponding to device activity at each point in the scanned area.
While the above systems and methods provide valuable information about the functionality of the DUT, it is desirable to non-invasively obtain further information about the response of various active devices within the DUT.